The present invention relates generally to nip presses, and more particularly to shoe presses.
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper xe2x80x9cstockxe2x80x9d) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a xe2x80x9cforming fabric,xe2x80x9d provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the xe2x80x9cmachine sidexe2x80x9d) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more presses (often roller presses) covered with another fabric, typically referred to as a xe2x80x9cpress felt.xe2x80x9d Pressure from the presses removes additional moisture from the web; the moisture removal is often enhanced by the presence of a xe2x80x9cbattxe2x80x9d layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Over the last 25 or 30 years, a xe2x80x9cshoe pressxe2x80x9d has been developed for the press section of the papermaking machine. A shoe press includes a roll or similar structure that mates with a xe2x80x9cshoexe2x80x9d of an opposed roll or press structure; the surface of the shoe is somewhat concave and approximates in curvature the convex profile of the mating roll. This arrangement can increase the width of the nip in the direction of paper travel, thereby enabling greater amounts of water to be removed therein.
Endless belts or blankets have traditionally been used in shoe press operations. The belt overlies and contacts the shoe of the press; in turn, the press felt overlies the shoe press belt, and the paper web overlies the press felt The shoe press belt and press felt travel through the nip and, in doing so, convey the paper web through the nip. The press felt travels over a set of rollers arranged around the shoe. In older embodiments, shoe press belts were also driven by sets of drive rollers arranged around the shoe. In some newer configurations, however, the shoe press belt is clamped or otherwise fixed to the edges of circular head plates located on either end of the shoe, such that rotation of the head plates causes the shoe press belt to rotate and travel through the nip.
Given the performance requirements, a shoe press belt should be sufficiently flexible to pass around the drive rollers or head plates and through the shoe and sufficiently durable to withstand the repeated application of pressure within the nip. Because of these performance parameters, most endless belts are formed entirely or predominantly of a polymeric material (often polyurethane). Many shoe press belts also include reinforcing fibers or a reinforcing fabric between or embedded in polymeric layers. Also, shoe press belts may be configured to encourage water to pass from the paper web. To this end, some shoe press belts have grooves or blind-drilled holes in the surface adjacent the press felt that serve to vent water from the paper that is exiting the press felt.
As the paper web is conveyed through the nip, it can be very important to understand the pressure profile experienced by the paper web. Variations in nip pressure can impact the amount of water drained from the web, which can affect the ultimate sheet moisture content, thickness, and other properties. Excessive nip pressures can cause crushing or tearing of the web. Of course, in a shoe press the pressure typically varies at different locations in the nip, both along and transverse to the direction of paper travel, and can also vary over time. As a result, it would be desirable to have a reliable technique and apparatus for determining the pressure distribution and area of the nip in a shoe press.
Other properties of a shoe press belt can also be important. For example, the stress and strain experienced by the belt, both in the machine direction and the cross machine direction, can provide information about the durability and dimensional stability of the belt. In addition, the temperature profile of the belt can assist in identifying potential problem areas of the belt. As such, it would be desirable to have a reliable technique and apparatus for determining these properties of a shoe press belt.
The present invention is directed to a shoe press and associated belt that can determine operating parameters within the nip of a shoe press. A shoe press of the present invention comprises: a first member; a second member; a substantially cylindrical belt; and a processing unit. The first member has a convex pressing surface. The second member includes a shoe with a concave pressing surface substantially complimentary to the convex pressing surface. The second member further includes a pair of substantially circular head plates rotatably mounted on axially opposed ends thereof. The belt is fixed to, extends between, and is rotatable with the head plates such that a portion of the belt passes between the convex pressing surface and the concave pressing surface. The belt includes embedded therein a communications cable having a plurality of sensors configured to generate signals responsive to an operating parameter of the shoe press. The processing unit is in communication with the communications cable and processes signals generated by the sensors. Thus, signals generated by the sensors and processed by the processing unit represent conditions (particularly pressure, nip width, temperature, strain and stress) within the nip of the shoe press that can be displayed and understood by an operator.
In one embodiment, the belt comprises: a substantially cylindrical inner polymeric layer having a first longitudinal axis and a radially inner surface; a substantially cylindrical outer polymeric layer having a second longitudinal axis that is substantially collinear with the first axis and a radially outer surface; a substantially cylindrical fabric layer sandwiched between the inner and outer polymeric layers; and a communications cable having a plurality of sensors configured to detect an operating parameter of a shoe press. The radially inner and radially outer surfaces define a belt thickness, and the sensing fiber extends within the belt thickness. Preferably, the inner and outer polymeric layers are polyurethane, and the sensing fiber is an optical fiber that travels in a single helix along the length and circumference of the belt.